The paper “Virtual Synchronous Machine” by Hans-Peter Beck & Ralf Hesse; Proceedings Electrical Power Quality and Utilization—EPQU 2007, 9th International, 9-11 Oct. 2007, and WO 2009/022198 represent the available prior art. Hans-Peter et al. apply the term Virtual Synchronous Machine (VISMA), whereas WO 2009/022198 uses the term Conditioning Device for Energy Supply Networks. The latter is based on today's inverter technology where it is possible to specify the controller properties in such a way that it acts like a synchronous machine connected to a power grid.
The VISMA is based on the complete two-axis dynamic mathematical model of an electrically excited synchronous machine. This dynamic model is fully described by the d-q-axis electrical parameters of the stator, the exciter and the damper as well as by the magnetic coupling and the mass inertia of a virtual rotor. The full analogy with the synchronous machine is established on the basis of virtual values of torque and excitation voltage. Due to the analogy to a synchronous machine, conventional grid operation is possible in parallel with conventional synchronous machines.
In practice, the VISMA basically measures the terminal voltages at the point of connection with the grid and then calculates in real time the equivalent currents of a synchronous machine. Then the calculated machine currents are the reference values for a current controller which is controlling the injected currents into the grid by means of an inverter. The inverter must always be capable of feeding into the grid the current value calculated with the machine model otherwise the VISMA looses its linear properties.
To control the VISMA in analogy with the synchronous machine, the virtual shaft is provided with a virtual torque value for transferring active power to the grid. To change the transferred active power a change in the virtual torque must be done. Then the active power variation will follow the dynamics of the simulated rotor (rotor swings). The virtual shaft stabilizes at a new angle, said angle being associated with a slow dynamic and overshot in the active power.
Also a virtual excitation is provided that can be used for terminal voltage control. Sufficient computational capacity is necessary to build the real time machine model.
It is an object of the present invention to provide a faster and a simpler way for emulating a synchronous machine while maintaining the possibility for fast and decoupled following of set points for generated active and/or reactive powers.